Power semiconductor modules comprise a semiconductor package and at least two power semiconductor chips mounted on one or more substrates within the package. The power semiconductor chips usually comprise power electronic circuits such as rectifier bridges, DC-links, IGBT inverters, drivers, control units, sensing units, half bridge inverters, AC-DC converters, DC-AC converters, DC-DC converters, bidirectional hybrid switches, and more.
In case of a plurality of substrates, interconnections from one substrate to another are provided within the package. As shown in FIG. 1, a power semiconductor module 10 comprises as a substrate an insulating ceramic plate 14 having a metallization 15 and 16 on each side of the ceramic plate, but in some case, only one metallization 15 may be sufficient. At the top side, the power semiconductor chips 12 are joined to the substrate by any known joining mechanism, e.g., soldering 13. At the bottom side, the substrate is joined to a base plate 18 by any known joining mechanism, e.g., soldering 17. As the power electronic circuits 12 of the power semiconductor module generate heat during operation, the module needs to be mounted on a cooling arrangement 19, such as a heat exchange arrangement or a heat sink for example. Between heat sink and module thermally conductive grease is applied.
Accordingly, efficient heat transfer between the module and the cooling arrangement is required. For this reason, many kinds of material are used for forming the different plates, or layers, of the module, as well as various assembling techniques for joining such plates together. For example, ceramics such as Al2O3, AlN, Si3N4 may be used as the insulating material for the substrate, and copper or aluminium is bonded to the ceramic by the known Direct Copper Bonding (DCB), Active Metal Brazing (AMB), or Direct Aluminium Bonding (DAB) methods. Copper thickness ranges, for example, from 0.1 mm to 0.6 mm and the ceramic thickness ranges, for example, from 0.2 mm to 2 mm. If the substrate is soldered to the base plate, a copper or aluminium metallization is formed on both sides of the ceramic substrate. Depending on the application of the power semiconductor module, substrates can also be Ni-, Ag-, Pd-, Pt-, W-, Mo-, Mg-, Au-, Ti, Cr-, Al- (in case of Cu metallization for gluing) plated. Combinations of these materials in multilayer plating like Ni/Au, NiPdAg-, Au-, Ni/Ag-, Ni/Pd-, NiPdAg-, Ni/Au-plating, etc. are also applied. If the metallization is aluminium, plating like Ni-, Ni/Ag-, Cu-, Cu/Ni/Au-, Cu/Ag-, Ni/Pd-, NiPdAg-, Ni/Au-, Ti/Ni-, Ti/Ni/AgCr/Ni-, Cr/Ni/Ag-plating is a requirement in case of soldering.
In operation of the power semiconductor modules, the joining layers between the chips and the substrate(s), and that between the substrate(s) and the base plate are subject to thermal-mechanical stress. The modules are expected to withstand a high number of thermal cycles, i.e., the number of ups and downs of temperature over the respective temperature swing. Power semiconductor chips operate at junction temperatures of typically below 125° C. or 150° C. Junction temperature is the temperature of the semiconductor die within a semiconductor device package, e.g. a module.
However, certain applications such as automotive applications require a junction temperature higher than that of conventional cases. For instance, in hybrid vehicles, since it is wished to use the coolant for cooling the combusting engine also for cooling the power semiconductor modules, the junction temperature for the power semiconductor chips may be as high as 175° C. or even 200° C. A result of this high junction temperature is that, at the substrate, the operation temperature is typically around 110° C. and can reach up to 140° C. Therefore, the joining layer between the substrate and the base plate would be subject to this substrate temperature on the substrate side and to the temperature of the coolant on the baseplate side. To be exact, the base-plate side of the joining layer is subject to the temperature of the coolant plus some 10° C., which results from the thermal impedance from the substrate to the base plate and coolant. Consequently, it is expected that, in hybrid electric vehicles, the joining layer between the substrate and the base plate could experience a far wider range of temperature than in the case of conventional applications—the temperature swing here can be 30° C. to 60° C. more, which is roughly double of the temperature swing as found in conventional cases. Since the life time of the module in number of temperatures cycles decreases almost exponentially with the temperature swing and/or according to 1/ΔTx, with x>1, the number of achievable temperature cycles of the power semiconductor module would be significantly reduced in such automotive application.
Accordingly, there is a need to extend the lifetime of such modules.